U.S. patent number 7,498,092 [Application Number 11/044,283] was granted by the patent office on 2009-03-03 for perpendicular magnetic recording medium with magnetic torque layer coupled to the perpendicular recording layer.
This patent grant is currently assigned to Hitachi Global Storage Technologies Netherlands B.V.. Invention is credited to Andreas Klaus Berger, Hoa Van Do.
United States Patent |
7,498,092 |
Berger , et al. |
March 3, 2009 |
Perpendicular magnetic recording medium with magnetic torque layer
coupled to the perpendicular recording layer
Abstract
A perpendicular magnetic recording medium, such as a
perpendicular magnetic recording disk, has a magnetic "torque"
layer (MTL) that exerts a magnetic torque onto the perpendicular
magnetic recording layer (RL) in the presence of the applied
perpendicular write field. The MTL thus acts as a write assist
layer in reversing the magnetization of the RL. A coupling layer
(CL) is located between the MTL and the RL and provides the
appropriate ferromagnetic coupling strength between the MTL and the
RL.
Inventors: |
Berger; Andreas Klaus (San
Jose, CA), Do; Hoa Van (Fremont, CA) |
Assignee: |
Hitachi Global Storage Technologies
Netherlands B.V. (Amsterdam, NL)
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Family
ID: |
35759099 |
Appl.
No.: |
11/044,283 |
Filed: |
January 26, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060166039 A1 |
Jul 27, 2006 |
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Current U.S.
Class: |
428/829; 428/828;
428/828.1; 428/827 |
Current CPC
Class: |
G11B
5/7369 (20190501); G11B 5/7368 (20190501); G11B
5/66 (20130101); G11B 5/656 (20130101) |
Current International
Class: |
G11B
5/66 (20060101) |
Field of
Search: |
;428/827,828 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1453038 |
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Sep 2004 |
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EP |
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2004/097809 |
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Nov 2004 |
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WO |
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Other References
EPO Search Report for the counterpart EPO application. cited by
other .
Ando, et al., "Effects of In-Plane Hard Magnetic Layer on
Demagnetization and Media Noise in Triple-Layered Perpendicular
Recording Media", IEICE Trans Electron., vol. E78-C, No. 11, Nov.
1995. cited by other .
Benakli et al., "Micromachined Study of Switching Speed in
Perpendicular Recording Media", IEEE Trans on Magnetics, vol. 37,
No. 4, Jul. 2001, pp. 1564-1566. cited by other .
Gao et al., "Transition Jitter Estimates in Tilted and Conventional
Perpendicular Recording Media at 1 Tb/in2", IEEE Trans on
Magnetics, vol. 39, No. 2, Mar. 2003, pp. 704-709. cited by
other.
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Primary Examiner: Tarazano; D. Lawrence
Assistant Examiner: Harris; Gary D
Attorney, Agent or Firm: Berthold; Thomas R.
Claims
What is claimed is:
1. A perpendicular magnetic recording medium for use in a system
with a perpendicular recording write pole, the medium comprising: a
substrate; an underlayer of magnetically permeable material on the
substrate as a return path for magnetic flux from the write pole;
an exchange break layer on the underlayer; a ferromagnetic write
assist layer having an easy axis of magnetization and a
magnetization less than about 45 degrees out-of-plane; a
ferromagnetic recording layer having perpendicular magnetic
anisotropy and an out-of-plane easy axis of magnetization
substantially perpendicular to the plane of the recording layer and
non-collinear with the easy axis of magnetization of the write
assist layer; a nonmagnetic coupling layer between the write assist
layer and the recording layer and having a thickness between about
5 .ANG. and 30 .ANG., the coupling layer being in direct contact
with the write assist layer and the recording layer and permitting
ferromagnetic coupling of the write assist layer with the recording
layer, wherein the write assist layer, coupling layer and recording
layer are located on the exchange break layer; and wherein the
exchange break layer between the underlayer and the recording layer
prevents magnetic exchange coupling between the underlayer and the
recording layer.
2. The medium of claim 1 wherein the write assist layer is a
hexagonal-close-packed material with its c-axis oriented
substantially perpendicular to the write assist layer and the
coupling layer is a hexagonal-close-packed material with its c-axis
oriented substantially perpendicular to the coupling layer.
3. The medium of claim 1 wherein the write assist layer is a
magnetically permeable material with coercivity less than about
2000 Oe.
4. The medium of claim 1 wherein the write assist layer is formed
of a material selected from the group consisting of (a) an alloy
comprising Co and one or more of Cr, B, Ru, and Ta, and (b) an
alloy comprising Co, Pt, Cr and B with Pt content less than about
12 atomic percent.
5. The medium of claim 4 wherein the write assist layer further
comprises one or more oxides of Si, Ta, Ti and B.
6. The medium of claim 1 wherein the coupling layer is formed of a
material selected from the group consisting of (a) a RuCo alloy
with Co less than about 40atomic percent, (b) a RuCoCr alloy with
Co less than about 40 atomic percent, and (c) an alloy of Co and
one or more of Cr and B with the combined content of Cr and B
greater than about 30 atomic percent.
7. The medium of claim 6 wherein the coupling layer further
comprises one or more oxides of Si, Ta, Ti and B.
8. The medium of claim 1 wherein the write assist layer is on the
exchange break layer, the coupling layer is on the write assist
layer and the recording layer is on the coupling layer.
9. The medium of claim 1 wherein the coupling layer is on the
recording layer and the write assist layer is on the coupling
layer.
10. The medium of claim 1 wherein the exchange break layer consists
essentially of Ru and further comprising a seed layer consisting
essentially of NiFe between the underlayer and the Ru exchange
break layer.
11. The medium of claim 1 wherein the exchange break layer consists
essentially of titanium.
12. The medium of claim 1 wherein the exchange break layer is
formed of material selected from the group consisting of Si, Ge,
SiGe alloys, Cr, Ru, W, Zr, Nb, Mo, V, Al, CrTi, NiP, CN.sub.x,
CH.sub.x, C, and oxides, nitrides and carbides of an element
selected from the group consisting of Si, Al, Zr, Ti, and B.
13. The medium of claim 1 wherein the recording layer is a granular
polycrystalline cobalt alloy.
14. The medium of claim 13 wherein the recording layer further
comprises an oxide of one or more of Si, Ta, Ti and B.
15. The medium of claim 1 wherein the recording layer is a
multilayer selected from the group consisting of Co/Pt, Co/Pd,
Fe/Pt and Fe/Pd multilayers.
16. The medium of claim 1 wherein the underlayer is formed of a
material selected from the group consisting of alloys of CoFe,
CoNiFe, NiFe, FeCoB, CoCuFe, FeAlSi, FeTaN, FeN, FeTaC, CoTaZr and
CoZrNb.
17. The medium of claim 1 wherein the underlayer is a lamination of
multiple magnetically permeable films separated by nonmagnetic
films.
18. The medium of claim 17 wherein the nonmagnetic films in the
lamination provide antiferromagnetic coupling of the magnetically
permeable films in the lamination.
19. A perpendicular magnetic recording disk for use in a disk drive
with a perpendicular recording write pole, the disk comprising: a
substrate; an underlayer of magnetically permeable material on the
substrate as a return path for magnetic flux from the write pole;
an exchange break layer on the underlayer; a ferromagnetic write
assist layer on the exchange break layer and having an easy axis of
magnetization and a magnetization less than about 45 degrees
out-of-plane; a ferromagnetic recording layer having perpendicular
magnetic anisotropy and an out-of-plane easy axis of magnetization
substantially perpendicular to the plane of the recording layer and
non-collinear with the easy axis of magnetization of the write
assist layer, the exchange break layer preventing magnetic exchange
coupling between the underlayer and the recording layer; and a
coupling layer between the write assist layer and the recording
layer and having a thickness between about 5 .ANG. and 30 .ANG.,
the coupling layer being in direct contact with the write assist
layer and the recording layer and permitting ferromagnetic coupling
of the write assist layer with the recording layer, the coupling
layer being formed of a material selected from the group consisting
of (a) a RuCo alloy with Co less than about 40 atomic percent, (b)
a RuCoCr alloy with Co less than about 40 atomic percent, and (c)
an alloy of Co and one or more of Cr and B with the combined
content of Cr and B greater than about 30 atomic percent.
20. The disk of claim 19 wherein the write assist layer is a
hexagonal-close-packed material with its c-axis oriented
substantially perpendicular to the write assist layer and the
coupling layer is a hexagonal-close-packed material with its c-axis
oriented substantially perpendicular to the coupling layer.
21. The disk of claim 19 wherein the write assist layer is a
magnetically permeable material with coercivity less than about
2000 Oe.
22. The disk of claim 19 wherein the write assist layer is formed
of a material selected from the group consisting of (a) an alloy
comprising Co and one or more of Cr, B, Ru, and Ta, and (b) an
alloy comprising Co, Pt, Cr and B with Pt content less than about
12 atomic percent.
23. The disk of claim 22 wherein the write assist layer further
comprises one or more oxides of Si, Ta, Ti and B.
24. The disk of claim 19 wherein the coupling layer further
comprises one or more oxides of Si, Ta, Ti and B.
25. The disk of claim 19 wherein the exchange break layer consists
essentially of Ru and further comprising a seed layer consisting
essentially of NiFe between the underlayer and the Ru exchange
break layer.
26. The disk of claim 19 wherein the exchange break layer consists
essentially of titanium.
27. The disk of claim 19 wherein the exchange break layer is formed
of material selected from the group consisting of Si, Ge, SiGe
alloys, Cr, Ru, W, Zr, Nb, Mo, V, Al, CrTi, NiP, CN.sub.x,
CH.sub.x, C, and oxides, nitrides and carbides of an element
selected from the group consisting of Si, Al, Zr, Ti, and B.
28. The disk of claim 19 wherein the recording layer is a granular
polycrystalline cobalt alloy.
29. The disk of claim 28 wherein the recording layer further
comprises an oxide of one or more of Si, Ta, Ti and B.
30. The disk of claim 19 wherein the recording layer is a
multilayer selected from the group consisting of Co/Pt, Co/Pd,
Fe/Pt and Fe/Pd multilayers.
31. The disk of claim 19 wherein the underlayer is formed of a
material selected from the group consisting of alloys of CoFe,
CoNiFe, NiFe, FeCoB, CoCuFe, FeAlSi, FeTaN, FeN, FeTaC, CoTaZr and
CoZrNb.
32. The disk of claim 19 wherein the underlayer is a lamination of
multiple magnetically permeable films separated by nonmagnetic
films.
33. The disk of claim 32 wherein the nonmagnetic films in the
lamination provide antiferromagnetic coupling of the magnetically
permeable films in the lamination.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to perpendicular magnetic
recording media, and more particularly to a disk with a
perpendicular magnetic recording layer for use in magnetic
recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are
stored in a perpendicular or out-of-plane orientation in the
recording layer, is a promising path toward ultra-high recording
densities in magnetic recording hard disk drives. The most common
type of perpendicular magnetic recording system is one that uses a
"probe" or single pole recording head with a "dual-layer" media as
the recording disk. The dual-layer media comprises a perpendicular
magnetic data recording layer (RL) formed on a "soft" or relatively
low-coercivity magnetically permeable underlayer (SUL), with the
SUL serving as a flux return path for the field from the pole
recording head. This type of system is also called "Type 1"
perpendicular magnetic recording. A schematic of such a prior art
system with a read element for reading the recorded data is shown
in FIG. 1.
FIG. 2 is a schematic of a cross-section of a prior art
perpendicular magnetic recording disk showing the write field H
acting on the recording layer RL. The disk also includes the hard
disk substrate, a seed or onset layer (OL) for growth of the SUL,
an exchange break layer (EBL) to break the magnetic exchange
coupling between the magnetically permeable films of the SUL and
the RL and to facilitate epitaxial growth of the RL, and a
protective overcoat (OC). As shown in FIG. 2, the RL is located
inside the gap of the "apparent" recording head (ARH), which allows
for significantly higher write fields compared to longitudinal or
in-plane recording. The ARH comprises the write pole (FIG. 1) which
is the real write head (RWH) above the disk, and a secondary write
pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which
is decoupled from the RL by the EBL and produces a magnetic mirror
image of the RWH during the write process. This effectively brings
the RL into the gap of the ARH and allows for a large write field H
inside the RL. However, this geometry also results in the write
field H inside the RL being oriented nearly parallel to the surface
normal, i.e., along the perpendicular easy axis of the RL grains,
as shown by typical grain 10 with easy axis 12. The parallel
alignment of the write field H and the RL easy axis has the
disadvantage that relatively high write fields are necessary to
reverse the magnetization because no torque is exerted onto the
grain magnetization. Also, a write-field/easy-axis alignment
increases the magnetization reversal time of the RL grains (M.
Benakli et al., IEEE Trans. MAG 37, 1564(2001)). For these reasons,
"tilted" media have been theoretically proposed, in which the
magnetic easy axis of the RL is tilted at an angle of about 45
degrees with respect to the surface normal, so that magnetization
reversal can be accomplished with a lower write field and without
an increase in the reversal time. (K.-Z. Gao and H. N. Bertram,
IEEE Trans. MAG 39, 704(2003)). However, there is no known
fabrication process to make high-quality recording media with a
tilted easy axis.
What is needed is a perpendicular magnetic recording media that
displays a magnetization reversal behavior similar to tilted media
and is fully compatible with conventional fabrication
processes.
SUMMARY OF THE INVENTION
The invention is a perpendicular magnetic recording medium, such as
a perpendicular magnetic recording disk, with a magnetic "torque"
layer (MTL) that exerts a magnetic torque onto the perpendicular
magnetic recording layer (RL) in the presence of the applied
perpendicular write field. The MTL thus acts as a write assist
layer in reversing the magnetization of the RL. A coupling layer
(CL) is located between the MTL and the RL and provides the
appropriate ferromagnetic coupling strength between the MTL and the
RL. The MTL has a substantial in-plane magnetization component and
an easy axis of magnetization that is non-collinear with the RL
easy axis of magnetization. In one embodiment the MTL is located
between the soft magnetic underlayer (SUL) of the disk and the RL,
and the CL is located between the MTL and the RL. In this
embodiment the MTL and CL are formed of materials that enable the
growth of a high performance RL on top of the MTL/CL structure. In
a second embodiment the CL is located on top of the RL and the MTL
is located on top of the CL.
While the primary purpose of the MTL is the application of torque
during the write process, which aids the overall write process, the
MTL may also exhibit a magnetization pattern that adds to the
overall media signal, even in the absence of an externally applied
field.
For a fuller understanding of the nature and advantages of the
present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a prior art perpendicular magnetic
recording system.
FIG. 2 is a schematic of a cross-section of a prior art
perpendicular magnetic recording disk showing the write field H
acting on the recording layer (RL).
FIG. 3 is a schematic of a cross-section of the perpendicular
magnetic recording disk according to a first embodiment of the
present invention showing the write field H acting on the recording
layer (RL) and the magnetic torque layer (MTL) coupled to it by a
ferromagnetic coupling layer (CL), with the CL and MTL being
located below the RL.
FIG. 4 is a schematic of a magnetic model of the magnetic grains of
the recording layer (RL) and magnetic torque layer (MTL) of the
present invention with their respective easy axes of
magnetization.
FIG. 5 is a graph of calculated switching field H.sub.S in units of
H.sub.K as a function of the grain orientation angle .theta. for
various values of the coupling strength J.
FIG. 6 shows the distribution of switching field H.sub.S values for
a Gaussian distribution of grain orientation angles .theta.,
centered around .theta.=0 with a full-width-half-maximum (FWHM) of
3.3 degrees, for the disk structure of the invention with an MTL
coupled to the RL (J=0.05) and for the prior art disk structure
without an MTL (J=0).
FIG. 7 is a schematic of a cross-section of the perpendicular
magnetic recording disk according to a second embodiment of the
present invention showing the write field H acting on the recording
layer (RL) and the magnetic torque layer (MTL) coupled to it by a
ferromagnetic coupling layer (CL), with the CL and MTL being
located above the RL.
DETAILED DESCRIPTION OF THE INVENTION
The invention is a perpendicular magnetic recording medium with a
thin magnetic "torque" layer (MTL) that has a substantial in-plane
magnetization component and an easy axis of magnetization that is
non-collinear with the RL easy axis of magnetization, and a
ferromagnetic coupling layer (CL) between the MTL and RL that
provides the appropriate ferromagnetic coupling strength between
MTL and RL. A MTL with an in-plane easy axis of magnetization is
one particular implementation of the invention. The medium in the
form of a disk is shown in sectional view in FIG. 3 with the write
field H. In the embodiment of FIG. 3, the CL and MTL are located
below the RL. As shown in the exploded portion of FIG. 3, a typical
grain 10 in the RL has a perpendicular or out-of-plane
magnetization along easy axis 12, and a typical grain 20 in the MTL
directly below RL grain 10 has a substantially horizontal or
in-plane magnetization along easy axis 22. The magnetization of the
MTL does not need to be entirely in-plane but should have a
substantial in-plane component, such as would be provided if the
magnetization of the MTL is less than about 45 degrees
out-of-plane. In the presence of the applied perpendicular write
field H, the MTL acts as a write assist layer by exerting a
magnetic torque onto the RL that assists in reversing the
magnetization of the RL. While the primary purpose of the MTL is
the application of torque during the write process, which aids the
overall write process, the MTL may also exhibit a magnetization
pattern that adds to the overall media signal, even in the absence
of an externally applied field.
The advantage of the medium of this invention has been estimated by
magnetic modeling calculations. In the model, schematically
displayed in FIG. 4, the MTL-RL grain is modeled as two coupled
Stoner-Wohlfarth particles, each having uniaxial magnetic
anisotropy but oriented orthogonal to one another. Uniaxial
magnetic anisotropy of a ferromagnetic layer with an anisotropy
constant K means essentially that all of the magnetic moments tend
to align along the same axis, referred to as the easy axis, which
is the lowest energy state. The anisotropy field H.sub.k of a
ferromagnetic layer with uniaxial magnetic anisotropy K is the
magnetic field that would need to be applied along the easy axis to
switch the magnetization direction.
In the example that was modeled, the RL grain was characterized by
magnetization M.sub.1, anisotropy constant K.sub.1 and volume
V.sub.1, while the MTL grain was characterized by magnetization
M.sub.1, anisotropy constant -K.sub.1 and volume 0.2V.sub.1. The
opposite signs for the anisotropy constants represent the
orthogonal magnetic anisotropy. The MTL was assumed to be a factor
of 5 thinner than the RL to achieve a realistic geometry for the
medium structure. For a thicker MTL, a larger torque and write
assist effect would be expected, but other recording properties
might degrade if the MTL is too thick. The MTL and RL were coupled
across the CL by a ferromagnetic coupling of strength J, whose size
is given in fractions of the anisotropy energy of the RL grain,
i.e., J=xK.sub.1V.sub.1. In the modeled calculation, this coupled
grain system was exposed to a static magnetic field H applied at an
angle .theta. with the grain axis, and the magnetic switching field
H.sub.S was calculated.
FIG. 5 shows the calculated switching field H.sub.S in units of
H.sub.K (=2K.sub.1/M.sub.1) as a function of the grain orientation
angle .theta. for various values of the coupling strength J. The
seven curves represent the coupling values J=0 to 0.06 in steps of
0.01. Model calculations for J=0 are equivalent to the case of a RL
without an MTL. As can be seen from FIG. 5, coupling of the RL to
the MTL lowers the switching field H.sub.S, in particular for
well-oriented grains near .theta.=0. For example, with J=0.06, at
.theta.=0 the switching field H.sub.S is only approximately 75% of
H.sub.K, which would be the switching field required without the
MTL.
Because it is not possible to manufacture a disk in which the grain
orientation .theta. is a constant value at all locations on the
disk, it is important to also model the invention for a
distribution of .theta. values. FIG. 6 shows as an example the
distribution in values of switching field H.sub.S for a Gaussian
distribution of .theta., centered around .theta.=0 with a FWHM of
3.3 degrees, for the disk structure of the invention with an MTL
coupled to the RL (J=0.05) and for the prior art disk structure
without an MTL (J=0). It is evident that coupling to the MTL
significantly lowers both the mean switching field <H.sub.S>
and the switching field distribution width .DELTA.H.sub.S. Thus,
MTL-type media are expected to not only show improved writability
but also lower intrinsic media noise, since it is known that a
large switching field distribution is a significant contributor to
media jitter. The modeling has also shown that for the same
distribution of .theta. (e.g., for .theta. centered around 0 with a
FWHM of 3.3 degrees, as for FIG. 6), both the mean switching field
<H.sub.S> and the relative switching field distribution width
.DELTA.H.sub.S/<H.sub.S> are decreased as J is increased.
Substantial improvement of up to a 20% reduction in <H.sub.S>
and 45% in .DELTA.H.sub.S/<H.sub.S> were observed. The
modeling has also established that the improvements are larger for
media with better grain orientation, i.e., MTL-type structures are
expected to be especially effective for the highest quality RL.
The calculated improvements are due to reduced activation barriers
of recording media at high applied write fields, which are achieved
by the torque of the MTL coupled to the RL. However, the
calculations did not consider the true dynamic torques occurring in
such geometry and their effect on switching speed. Those effects
may be even larger than the calculated effects, which would
correspond to an even larger improvement potential for MTL-type
media.
A representative disk structure for the invention shown in FIG. 3
will now be described. The hard disk substrate may be any
commercially available glass substrate, but may also be a
conventional aluminum alloy with a NiP surface coating, or an
alternative substrate, such as silicon, canasite or
silicon-carbide.
The adhesion layer or OL for the growth of the SUL may be an AlTi
alloy or a similar material with a thickness of about 2-5 nm. The
SUL may be formed of magnetically permeable materials such as
alloys of CoNiFe, FeCoB, CoCuFe, NiFe, FeAlSi, FeTaN, FeN, FeTaC,
CoTaZr, CoFeB, and CoZrNb. The SUL may also be a laminated or
multilayered SUL formed of multiple soft magnetic films separated
by nonmagnetic films, such as electrically conductive films of Al
or CoCr. The SUL may also be a laminated or multilayered SUL formed
of multiple soft magnetic films separated by interlayer films that
mediate an antiferromagnetic coupling, such as Ru, Ir, or Cr.
The EBL is located on top of the SUL. It acts to break the magnetic
exchange coupling between the magnetically permeable films of the
SUL and the RL and also serves to facilitate epitaxial growth of
the RL. The EBL may not be necessary, but if used it can be a 5 nm
nonmagnetic titanium (Ti) layer; a non-electrically-conducting
material such as Si, Ge and SiGe alloys; a metal such as Cr, Ru, W,
Zr, Nb, Mo, V and Al; a metal alloy such as amorphous CrTi and NiP;
an amorphous carbon such as CN.sub.x, CH.sub.x and C; or oxides,
nitrides or carbides of an element selected from the group
consisting of Si, Al, Zr, Ti, and B. If an EBL is used, a seed
layer may be used on top of the SUL before deposition of the EBL.
For example, if Ru is used as the EBL, a 2-4 nm thick NiFe seed
layer may be deposited on top of the SUL, followed by a 5-20 nm
thick Ru EBL.
The RL can be any of the known amorphous or crystalline materials
and structures that exhibit perpendicular magnetic anisotropy.
Thus, the RL may be a layer of granular polycrystalline cobalt
alloy, such as a CoPt or CoPtCr alloy, with or without an oxide,
such as oxides of Si, Ta, Ti, or B. Also, the RL may be composed of
multilayers with perpendicular magnetic anisotropy, such as Co/Pt,
Co/Pd, Fe/Pt and Fe/Pd multilayers. In addition, perpendicular
magnetic layers containing rare earth elements are useable for the
RL, such as CoSm, TbFe, TbFeCo, GdFe alloys. The RL has a typical
thickness of 10-25 nm. The OC formed on top of the RL may be an
amorphous "diamond-like" carbon film or other known protective
overcoats, such as Si-nitride.
In the embodiment of FIG. 3, in which the MTL and CL are below the
RL, the MTL should be formed of a material that enables the growth
of a high performance RL on top of the MTL/CL structure. For
instance, the MTL could be a hexagonal-close-packed (hcp) material
with a basal plane magnetic easy axis. The c-axis of the MTL is
perpendicular or out-of-plane to encourage coherent epitaxial
growth of the CL and RL with their c-axes also perpendicular. This
material should be compatible with the EBL material on which it
grows and provide a good template for the CL and the RL. The
magnetic easy axis of the MTL in the basal plane does not have to
be due to the magnetocrystalline anisotropy of the MTL material
alone, because the MTL is exposed to a considerable magnetostatic
effect. Thus, the MTL will exhibit a remanent magnetic state with a
magnetization substantially in-plane (i.e., less than 45 degrees
out-of-plane) as long as the magneto-static effect is larger than
the magnetocrystalline anisotropy K, i.e., 2.pi.M.sub.S.sup.2>K,
because the effective anisotropy K.sub.eff is the difference of
both contributions K.sub.eff=K-(2.pi.M.sub.S.sup.2). A positive K
value refers herein to a material with a magnetocrystalline easy
axis along the out-of-plane c-axis. Thus, materials suitable for
the MTL are Co, CoCr, CoCrB, CoRu, CoRuCr, CoRuCrB, CoTa, and
CoPtCrB with low Pt content (less than about 12 atomic percent).
Si-oxide or other oxides like oxides of Ta, Ti, and B may be added
to these alloys in an amount up to about 15 atomic percent. The MTL
is a relatively soft ferromagnetic material with coercivity less
than approximately 2000 Oe. The MTL has a thickness of between
about 1-10 nm, preferably between about 1-5 nm.
In the embodiment of FIG. 3, in which the MTL and CL are below the
RL, the CL also has to sustain the growth of the RL, while
mediating a weak ferromagnetic coupling between the MTL and the RL.
Thus, hcp materials for instance, which can mediate a weak
ferromagnetic coupling, grow well on the potential MTL materials,
and provide a good template for the RL, are good candidates. Thus
the CL may be formed of RuCo and RuCoCr alloys with low Co content
(<about 40 atomic percent), or CoCr and CoCrB alloys with high
Cr and/or B content (Cr+B>about 30 atomic percent). Si-oxide or
other oxides like oxides of Ta, Ti and B may be added to these
alloys in an amount up to about 15 atomic percent. The CL has a
thickness of between about 0.5-10 nm, preferably between about
0.5-3 nm. The CL must enable an appropriate coupling strength.
Therefore, it needs to be either nonmagnetic or weakly
ferromagnetic. An appropriate coupling strength J, has to have a
considerable effect on the switching field (and the switching field
distribution), but has to be small enough so that it does not
couple the two layers rigidly together. Thus, an upper estimate for
the useful J range is K.sub.1*V.sub.1, with a preferred
implementation of J=0.03-0.4.
FIG. 7 shows a second embodiment of the invention in which the CL
and MTL are located above the RL. The CL and MTL function in the
same manner as described above. The embodiment of FIG. 3 provides
the advantage of locating the RL closer to the disk surface and
thus closer to the read/write head. However, if the CL and MTL are
located above the RL, as in FIG. 7, then there is less of a
requirement that the CL and MTL provide a good template for the
growth of the RL.
While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
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